The present invention relates to a light absorptive antireflector and a method for its production.
In recent years, with the rapid spread of computers, it has been demanded to reduce the reflection on a display surface and to provide an antistatic measure for the surface of a cathode ray tube (CRT) in order to improve the working environment for terminal operators. Recently, it has been further demanded to reduce the transmittance of a panel glass in order to improve the contrast and to shield electromagnetic waves of extremely low frequency.
In order to respond to such demands, methods have been adopted such that (1) an electrically conductive antireflection film is provided on a panel surface, (2) an electrically conductive antireflection film is provided on the front side of a face plate for a display screen of e.g. CRT, and such an antireflection film is bonded to the panel surface by means of a resin, and (3) a filter glass provided, on each side, with an electrically conductive antireflection film, is provided on the front side of CRT.
The following are known as conventional antireflection films. For example, xe2x80x9cOptical Thin Films User""s Handbookxe2x80x9d, J. D. Rancourt, McGRAW-HILL, pp128 (1987) discloses a spectral reflectance curve in a case where an absorptive film with a complex index of refraction (nxe2x88x92ik)=2xe2x88x922i and a transparent film with n=1.65 are formed on a substrate with a refractive index of 2.35 in this order in thicknesses of 3 nm and 75.8 nm, respectively.
However, in this case, presented are theoretically calculated values, and the reflection characteristics are reflection characteristics shown by a transparent double layer film as the basic construction for antireflection and correspond to a so-called xe2x80x9cV coatxe2x80x9d which makes the reflection to be 0 only at a single wavelength, and they are not ones which show a low reflection performance within a wide range of a wavelength region (such as from 500 to 650 nm).
Further, U.S. Pat. No. 5,091,244 discloses to form a transition metal nitride film and a transparent film in film thicknesses of from 6 to 9 nm and from 2 to 15 nm, respectively, sequentially from the substrate side, as a construction to reduce the reflection of incident light from the substrate side (incident light from the side opposite to the film surface side).
It is as disclosed, for example, in xe2x80x9cThin-Film Optical Filtersxe2x80x9d, H. A. Macleod, McGRAW-HILL, 2nd Ed., pp65-66 (1989) that when an absorptive film having a proper optical constant is formed to be thin, the reflectance from the substrate side decreases. In addition, in the proposal of the above-mentioned U.S. Pat. No. 5,091,244, SiO2 is formed to be thin (from 2 to 15 nm).
However, the construction of U.S. Pat. No. 5,091,244 wherein SiO2 is formed to be thin, is a construction by a film thickness designed for the purpose of reducing the reflection from the substrate side. In a case of a multilayer coating containing an absorptive film, reflection is totally different as between the front and rear sides, and accordingly, with this construction proposed to reduce the reflection from the substrate side, the reflectance from the film surface side will be about 10% over the visible light region, and a low reflection performance can not be obtained at all.
Further, the above-mentioned U.S. Pat. No. 5,091,244 illustrates a four layer construction of glass/transition metal nitride/transparent film/transition metal nitride/transparent film as a construction to reduce the reflection on the film surface side. However, the object is to control the visible light transmittance to a level of at most 50%, and this object is accomplished by dividing the absorptive layer into two layers to make the number of layers to be at least four layers, whereby there has been a practical problem from the viewpoint of the production cost.
JP-A-9-156964 discloses a construction having a transparent barrier film having a high refractive index provided between a light absorptive film and a low refractive index film in order to increase the resistance against oxidation of the light absorptive layer. However, with this construction, if the film thickness of the light absorptive film is made thick in order to lower the transmittance, the low reflection wavelength range tends to be narrow, and the bottom reflectance increases, whereby there has been a problem that the performance as an antireflection film is impaired.
Further, with the construction disclosed in the above-mentioned JP-A-9-156964 wherein a light absorptive film having a geometrical film thickness (hereinafter referred to simply as a film thickness) of from 15 to 30 nm, a transparent high refractive index film having a film thickness of from 10 to 40 nm and a silica film having a film thickness of from 50 to 90 nm are sequentially formed, the luminous reflectance shows a substantially low value, but reduction in the reflectance on a short wavelength side has been insufficient, and there has been a problem that the reflection color tends to be a strong blue, and the color can not be selected.
Further, with CRT in recent years, the transmittance of a glass base material for panel (hereinafter referred to simply as a panel base material) tends to be low year after year along with the requirement for high contrast, and a so-called xe2x80x9cdark tintxe2x80x9d panel has a transmittance of about 40%. When the transmittance of such a colored panel base material is combined with the antireflection film employing as a constituting element the absorptive film as disclosed in e.g. the above-mentioned JP-A-9-156964, the total transmittance will decrease. To reduce the total transmittance gives a preferred result for high contrast in many cases, but is not preferred since the initially planned transmittance (brightness) will change.
Namely, in designing the entire cathode ray tube, the final transmittance of the panel base material+the light absorptive antireflection film will be a question. The most preferred transmittance varies depending upon the type of the cathode ray tube, and it is difficult to realize the most preferred transmittance by limited types of panel base materials. Accordingly, it is desired to obtain a panel for cathode ray tube having a desired final transmittance by a combination of several kinds of panel base materials and by rather positively controlling the transmittance of the light absorptive antireflection film.
The above situation is not limited to a case where an antireflection film is applied directly to a panel glass for cathode ray tube but is likewise applicable to a case where, for example, a light absorptive antireflection film is applied to a plastic film, followed by bonding to a CRT panel glass later. Namely, in designing a final cathode ray tube, it is desired that the transmittance of the light absorptive antireflection film on the plastic film can optionally be selected.
It is an object of the present invention to solve the above-mentioned drawbacks of the prior art and to provide a light absorptive antireflector which has a sufficiently low reflection ability in a wide range of wavelength region, a sufficient low surface resistance to shield electromagnetic waves and a low luminous transmittance suitable for improving the contrast of a display and which has freeness in the transmittance and the reflection color and is excellent in the productivity, and a method for its production.
The present invention provide a light absorptive antireflector comprising a substrate, and a first layer of a light absorptive film, a second layer made of a material which has slight absorption within a wavelength of from 400 to 700 nm and which has an extinction coefficient being larger on a short wavelength side within a wavelength of from 400 to 700 nm and a third layer made of a material which is transparent within a wavelength of from 400 to 700 nm and which has a refractive index of less than 1.55, formed sequentially on the substrate, and a method for its production.
The slightly absorptive film to be used as the second layer in the present invention is effective to reduce the reflectance on the short wavelength side in a region with a wavelength of from 400 to 700 nm (hereinafter referred to also as a visible light region) (particularly effective in a case where the film thickness of the light absorptive film in the present invention is thick), and as compared with a case where a transparent film is used as the second layer, it is effective to sufficiently broaden the low reflection wavelength region and to realize a reflection spectrum which has the local maximum value in reflection at the center portion of the visible light region.